The Global Positioning System has become very valuable for communication, navigation and for geolocation. The GPS system includes about twenty-four spacecraft in 12-hour orbits at about 11000 nautical miles above the Earth. Each spacecraft includes a precision clock and a signal transmitter. A GPS receiver receives signals from several GPS spacecraft and measures the time of emission of the signal from each spacecraft. It then uses this information, together with knowledge of the location of the GPS satellites derived from the GPS signal, to calculate its own geolocation and time. Navigation systems exploit this knowledge to control vehicles and to target weapons. The GPS system also supports the synchronization of network systems such as communication systems.
One important aspect of GPS performance is the ability to predict each GPS space vehicle's (“SV”) position and clock state, as the broadcast message from each SV, which GPS users rely on to locate themselves, is based upon prediction. The dominant error in the range prediction error is the instability of the SV's clock. By enhancing the stability of each SV's clock, range prediction errors are reduced and the accuracy of the GPS system is improved.
A clock includes a periodic process and a counter. A simple clock produces a number in a register, which number corresponds to how many counts or how many periodic events have taken place since the clock was initialized. Most physical processes are not perfectly regular in time due to small physical noise processes that vary over time. Hence, clocks will deviate from “perfect” time, and the difference between absolute time and time measured by a clock will behave as a random process and the variation of the error will grow with time.
Prior to 1990, the GPS Master Control Station (GPS MCS) employed a “master clock” for time keeping. In the master clock implementation, a single clock is used to define time, and all other clocks are synchronized to the master clock through tracking these clocks from ground stations. Due to random variations and anomalies in the master clock, the master clock concept had its vulnerabilities. One source of these vulnerabilities of the GPS system is the necessity to keep GPS time close to coordinated universal time (UTC) time, which is Greenwich Mean Time updated with leap seconds. When the difference between the master clock and UTC became too large, then it was necessary to switch the master clock function from one clock to another. Changing the master clock introduced a discontinuity into the GPS service. The discontinuity disrupted service to those receivers that were in operation at the time of the discontinuity.
Since 1990, a composite clock, known as the Implicit Ensemble Mean (IEM), has replaced the master clock arrangement for GPS operations. The IEM composite clock uses a weighted linear combination of clocks, located both in space and on the ground to define “GPS time.” The IEM composite clock decreases the sensitivity of GPS time to the behavior of single clocks. The IEM composite clock applies weights associated with space and frequency estimates provided by a Kalman filter that solves for position of the GPS satellite in addition to solving for the clock states of the individual clocks. By changing the weights or contributions of the various clocks to GPS time, the GPS time can be gradually steered to UTC time without significant discontinuities. K. Brown, the developer of the IEM concept of “composite clock”, also points out that composite clocks provide better timing and geolocation accuracy than a master clock. Kenneth Brown, “The Theory of GPS Composite Clock”, Proceedings of 4th International Technical Meeting of the Satellite Division of the Institute of Navigation, Albuquerque, N. Mex., Sep. 11-13, 1991. Thus, by using a composite clock, the accuracy with which users determine their locations has improved. The argument for improved timing accuracy is that, if each clock is off by a random error, then averaging a group of clocks produces a more accurate estimation of time.
The IEM clock embedded within a filter as described by Brown assumes proportional process noise covariances. In this case, clocks have similarly shaped stability curves as a function of sample time.
The applicant's prior disclosure, entitled Stable Composite Clock, U.S. patent application Ser. No. 10/967,405 treats the case where not all clocks in a composite clock system have similarly shaped stability curves. That disclosure proposed applying Kalman filtering techniques to separate out phase and frequency clock noise contributions and produce a composite clock by combining these components into a composite clock frequency and phase, using different weights for the phase and frequency components. A special Kalman filter that solves for all clocks was disclosed. Inputs to this Kalman filter include the parameters that characterize Allan variance curves for all the individual clocks.
Both the IEM clock and the Optimum Stable Clock previously disclosed assume that all clocks are in communication with each other and/or a master clock. In GPS autonavigation, however, not all clocks are in communication with a master clock. Typically, each SV is in contact with about four other SVs. Thus, it is not possible to use existing methods to create a composite clock since all SV clock signals are not easily combined. Thus, a need exists for a method of improving the stability of a plurality of clocks by combining their signals when not all of the clocks are in communication with each other or with any one central clock. Such a method has application not only in the GPS system, but in any network containing multiple clocks wherein optimum clock stability is a critical aspect of the network.